Steric exclusion columns

The basic retention mechanisms of SEC were presented in Sections 16.3.3 and 16.3.4. These mainly include the steric exclusion of macromolecules from the pores of the column packing controlled... 

Separation is dependent on three different mechanims Donnan exclusion, steric exclusion, and adsorption/partitioning. Donnan exclusion causes strong acids to elute in the void volume of the column. Weak acids that are partially ionized in the eluent are not subject to the Donnan exclusion and can penetrate into the pores of the packing. Separation is achieved through differences in acid strength, size, and hydrophobicity. The major advantage of ion exclusion is the ability to handle samples that contain both strong and weak acids. 

FIGURE 3. Steric exclusion chromatography of water from waste trenches and inert atmosphere wells at Maxey Flats, sampled July, 1982. Water samples from waste trenches 27 and 19S (440 ml) and wells WIN (585 ml) and W2NA (238.5 ml) were concentrated to 10 ml, pH-adjusted to their original values and chromatographed on a Sephadex G-15 column at flow rates ranging from 28-33 ml/hr. Individual column fractions were collected every 10 min and analyzed for specific organic compounds and radionuclides. 

Di- and tricarboxylic acids such as oxalic and citric acid elute between the excluded and the total permeated volume. Apart from Donnan exclusion, the predominating separation mechanism is, in this case, mainly steric exclusion. The retention is determined by the size of the sample molecule. Since the pore volume of the resin is established by its degree of crosslinking, the resolution can only be improved by applying another or by coupling with another separator column, respectively. 

An extra group of special working procedures in gel chromatography are those separations where the steric exclusion mechanism is intentionally combined with an auxiliary additional separation mechanism to increase the selectivity. Such combinations can be realized either in one step, i.e., in one system gel-eluent, or in several steps, e.g., by the subsequent elution of the sample with two different mobile phases, or from two different column-filling materials. 

Large potentiality of the combination of different separation mechanisms within the same gel bed offer bidimensional columns [30], in which the bed is formed by a layer (as in TL GPC) in a sandwich arrangement. In one direction, the sample can be eluted in the steric exclusion mode and, using another eluent in the perpendicular direction, the elution is completed (as in the mode of sorption liquid chromatogra-phy). 

To illustrate the results obtainable in steric exclusion chromatography. Figure 21.24 depicts an HPLC separation of polystyrene standards in a column packed with cross-linked polystyrene particles (average pore size 260 A) of 10-/im d, using tetra-hydrofuran as a mobile phase. Unlike mobile phases in other forms of chromatog-... 

Kirkland, and Ely (24) have discussed the SEC technique. Barth (25) has reported a practical approach to steric exclusion chromatography of water-soluble polymers. However, SEC is not easily carried out for the subject polymers because of the high molecular weight (10 -10 g/mole) and the poly-electrolyte characteristics of the charged polymers. In order to obtain meaningful SEC data, the columns, mobile phase, concentration of polymer solution, sample preparation method, flow rate, and shear degradation of the polymer should be considered in an SEC experiment. 

In certain cases, a values of 30 or more have been found, which then correspond to A(AG) values in the range of 2 kcal/mol (8.4 kJ/mol). Generally, such values are obtained owing to very low retention of the first enantiomer eluted. This means that a very enantioselective sorption process is operating in the column, i.e., one of the enantiomers is virtually unbound by the CSP for steric reasons. Such phenomena are not easily explained by the three-point interaction model, but rather indicate the operation of a sort of chiral steric exclusion mechanism, more in line with a steric fit concept involving only one binding interaction. ... 

The separation mechanism in ion-exclusion chromatography is governed by Donnan exclusion, steric exclusion, sorption processes and, depending on the type of separator column, by hydrogen bonding. A high-capacity, totally sulfonated... 

Figure 1. Maltodextrins (M) and isomaltodextrins (IM) separation by steric exclusion chromatography on a Biogel P2 column (conditions given in Table I).

Warner, C. R., S. Selim, D. H. Daniels, Post-column complexation technique for the spectro-photometric detection of poly(oxy-l,2-ethanediyl) oligomers in steric exclusion chromatography,/. ChwmatogK, 1979,173, 357-363. 

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